In a mobile communications system, for an effective use of a radio wave resource or the like, an audio signal is required to be compressed to a low bit rate and transmitted. On the other hand, an increase in quality of a call voice and realization of the realistic high call service are also desired. To the realization, it is desirable to code not only a monaural signal but a multi channel acoustic signal, especially a stereo sound signal with high quality.
As a system for encoding a stereo sound signal with a low bit rate, an intensity stereo system has been known. The intensity stereo system employs a technique of multiplying a monaural signal by a scaling factor and generating an L-channel signal and an R-channel signal. Such a technique is also referred to as an amplitude panning.
The most fundamental technique of the amplitude panning is to multiply a monaural signal in a time domain by a gain coefficient for amplitude panning (panning gain coefficient) to obtain an L-channel signal and an R-channel signal (see, for example, non-patent literature 1). As another technique, in a frequency domain, a monophonic signal is multiplied by a panning gain coefficient for each frequency component or for each frequency group to obtain an L-channel signal and an R-channel signal (see, for example, non-patent literature 2).
If a panning gain coefficient is used as an encoding parameter of a parametric stereo, scalable encoding of a stereo signal (monophonic stereo scalable coding) is realizable (see, for example, patent literature 1 and patent literature 2). The panning gain coefficient is described as a balance parameter in a patent literature 1 and ILD (level difference) in patent literature 2, respectively.
When converting an acoustic signal into a frequency domain, generally a modified discrete cosine transform (hereinafter, described as “MDCT”) is used in consideration of characteristics of high conversion efficiency and difficulty in generation of high frame boundary distortion.